Conventionally, an underground survey has been conducted by an electromagnetic (EM) method or a magnetotelluric (MT) method.
In the EM method, an alternating current is allowed to flow through a transmission loop coil facing the ground to generate a primary magnetic field, and a secondary magnetic field induced by an eddy current generated under the ground due to the primary magnetic field is observed through a reception coil to measure electrical resistivity of the ground.
In the MT method, variations of an electric field and a magnetic field due to underground electromagnetic induction are observed through a grounded electrode and a reception coil, thereby measuring electrical resistivity of the ground. Although both methods are underground survey methods using electromagnetic induction, the EM method employs electrodes not buried in the ground. Another example of a method of estimating a distribution of electrical resistivity using an embedded electrode can include an electrical resistivity survey method. Such an EM method can provide easy measurement and is widely used to estimate a distribution of electrical resistivity under the ground.
In the EM method, a loop coil is carried by a person such that an underground survey can be easily conducted at lower cost. In addition, the EM method is little affected by topography and thus can easily determine two-dimensional diffusion and depth-wise variation of an electrical resistivity distribution, thereby allowing a three-dimensional survey.
However, the EM method is unsuitable for carrying out a wide-ranging underground survey despite the advantage of allowing an easy underground survey. Thus, there has been proposed a multi-frequency airborne electromagnetic method (for example, helicopter-borne electromagnetic method) wherein a loop coil is towed by a helicopter or the like. In this method, an alternating current is allowed to flow through a loop coil in the air such that a distribution of electrical resistivity under the ground can be measured using electromagnetic induction that occurs when an AC magnetic field moves under the ground.
Such an airborne electromagnetic survey is performed using a fixed-wing aircraft as shown in FIG. 2 or a rotary-wing aircraft as shown in FIG. 2 which flies along a predetermined flight path with an electromagnetic survey apparatus installed therein, and is essentially used in discovering overseas metal mineral resources due to effectiveness thereof in finding metal ores buried deep underground.
In a fixed-wing aircraft as shown in FIG. 1, electromagnetic waves are transmitted/received using a transmission coil disposed on the wings and fuselage and a reception coil placed under the aircraft, whereas, in a rotary-wing aircraft as shown in FIG. 2, electromagnetic waves are transmitted/received using a transmission coil and a reception coil each placed under the aircraft.
However, such an airborne electromagnetic survey is based on a high-priced fixed-wing aircraft or rotary-wing aircraft and is thus very costly. In addition, since it is very difficult and dangerous to add coils to a fixed wing of an aircraft designed to have an elaborate lift structure for stable flight, addition of coils to an aircraft not dedicated to an electromagnetic survey and use of such an aircraft in an electromagnetic survey are not preferred. Further, an electromagnetic survey using a rotary-wing aircraft also has a problem in that the aircraft should fly at low altitude with a bulky coil structure suspended therefrom, causing safety risks.
Particularly, in an airborne electromagnetic survey using a fixed-wing aircraft or a rotary-wing aircraft, a transmission coil and a reception coil are mainly disposed on the fuselage of an aircraft. Thus, signal interference often occurs due to a metal body constituting the fuselage or various electronic components during transmission/reception of electromagnetic waves, causing deterioration in reliability of measurement results.
In addition, when an aircraft designed to have a highly elaborate lift structure is used, placement or addition of a transmission coil and a reception coil is restricted. Thus, there is an efficiency problem in that it is necessary to fly the same course repeatedly with the transmission coil and the reception coil repositioned or added each time, depending on the purpose of a survey.
The present invention has been conceived to solve such problems in the art and it is an object of the present invention to provide an airship-based electromagnetic survey apparatus that includes a transmission coil and a reception coil disposed on an envelope of the airship so as to minimize signal interference due to a metal body constituting the airship and various electronic components while allowing placement of the transmission coil and the reception coil to be optimized for various survey purposes by the shape of the envelope such that various survey data can be acquired in just one flight.